1. Field of the Invention
The present invention relates generally to methods and apparatus for insulating well bores, and more particularly, but not by way of limitation, to methods and apparatus for insulating well bores with heat-activated gel-forming fluids.
2. Description of the Prior Art
It is often desirable and/or necessary to insulate well bores penetrating subterranean formations to reduce heat transfer between the well bores and surrounding formations. Generally, such well bores contain tubing strings extending from the surface to a point within the well bore adjacent the formation to be produced or into which fluids are to be injected. For example, in stimulating the recovery of oil from a heavy oil formation, i.e., a formation containing oil of high viscosity, steam flooding techniques are often utilized wherein steam is injected into the formation by way of one or more injection wells to heat the heavy oil and drive it towards and into one or more producing wells. In such steam stimulation operations, if the injected steam loses heat at a high rate to surrounding formations while flowing through the tubing string in an ejection well bore, the required or desired heat does not reach the formation and/or high energy consumption per barrel of oil produced results. The insulation of well bores to reduce heat transfer between the well bores and surrounding formations is often desirable in other applications such as in wells penetrating frozen strata (permafrost) to prevent melting, geothermal energy recovery wells to prevent the loss of heat from the fluids produced and in conventional wells wherein low strength or heat sensitive materials have been used.
Heretofore, well bores have been insulated by placing an insulating material in the annular space between the well bore and the tubing string disposed therein. For example, U.S. Pat. No. 3,451,479 dated June 24, 1969 to Parker teaches packing the annulus between a well bore and a tubing string with an aqueous solution of a water soluble inorganic salt, such as borax, sodium carbonate, sodium sulfate and mixtures thereof and then injecting a hot fluid through the tubing string into the formation to evaporate water from the solution whereby a substantial coat of the salt in solid form is deposited on the walls of the well bore and tubing string.
U.S. Pat. No. 3,861,469 dated Jan. 21, 1975 to Bayless et al. teaches the thermal insulation of a well bore by boiling a silicate solution in the annular space between the well bore and the tubing string to form a coating of alkali metal silicate foam on the tubing.
Other techniques have been used wherein gel-forming fluids and materials are gelled or solidified in the annuli of well bores to thermally insulate the well bores, but such techniques are generally expensive to carry out and/or the insulating solids produced are expensive or impossible to remove.
In the heretofore used techniques for insulating well bores, packers for providing leak-proof pressure seals between the tubing string and casing must be utilized to isolate the insulating medium introduced into the annulus from the well bore fluids below the packers. Typical such packers include elastomer sealing elements or mechanical sealing means which are often troublesome in high temperature environments and/or expensive to use. Generally, elastomer sealing elements deteriorate at high temperatures resulting in leaks and/or the packers becoming stuck in the well bore. In addition, packers designed for high temperatures are expensive and difficult to place in the well bore.
By the present invention improved methods of thermally insulating a well bore containing a tubing string to reduce heat transfer between the well bore and surrounding formations are provided wherein a heat-activated gel-forming fluid is introduced into and gelled in the annular space between the well bore and a tubing string disposed therein whereby the well bore is insulated. The methods are relatively inexpensive to carry out and the insulating gel produced can be removed from the well bore after use. Apparatus for carrying out the methods are also provided which are inexpensive and easily utilized. The apparatus does not include elastomer sealing members or mechanical seal means thereby obviating the problems associated therewith.